The invention is related to a pole-changing, three-phase winding with base-speed winding branches that are effective in both pole-number steps which are allocated, one third each, to all three winding phases and N dead branches that are effective only at one pole-pair number.
Three-phase windings of the above mentioned type are disclosed in U.S. Pat. No. 4,144,470 dated Mar. 13, 1979 and U.S. Pat. No. 4,284,919 dated Aug. 18, 1981. In these windings the base speed winding branches G are connected in three phase delta/three-phase delta, or three phase wye/three-phase delta. Alternatively, the base speed winding branches G can be connected in three-phase wye/three-phase wye in order to both reduce the connections to only six terminals and to require only a single three-phase changeover switch. One or several dead branches for at least one of the two pole-pair numbers can be connected to the base-speed winding in parallel and/or series. By means of imbrication of windings and variation of the number of turns per unit length favorable operating characteristics can be achieved in an electric machine.
The three-phase windings with three-phase wye/three-phase wye connection of the base-speed winding, disclosed in U.S. Pat. No. 4,144,470 and U.S. Pat. No. 4,284,919, can only be implemented for pole-pair ratios in accordance with ##EQU2## For the first pole-pair number p.sub.1 =3m.+-.1, not divisible by three, a distribution of coils is provided for each winding phase which is always grouped symmetrically along the periphery and which can be divided regularly into 2p.sub.1 /t equal-phase winding branches. For the second pole-pair number p.sub.2 =3n, divisible by three, the coils belonging to the individual winding phases of the first pole-pair number p.sub.1 =3m.+-.1, with regard to their phase relationship for the second pole number 2p.sub.2, are wound in layers without gaps and without overlapping over the entire periphery of the coil side star. The individual winding coils or coil groups are allocated in sectors, with .phi.:.psi.=G:N, to the different base-speed and dead branch windings.
Based on the three-phase-band winding configuration of p.sub.2 =3n, fully symmetrical m.m.f. characteristics for the pole-pair number divisible by three are achieved for known windings only with full-pitch windings for this pole-number step. For other coil-pitch increments, such as short-pitch windings, generally, only single-axis symmetrical m.m.f. polygons are achieved for p.sub.2 =3n or, as in the case of three-phase-band windings or imbricated windings with 2/3 wide phase spread, only three-axis periodical "Goerges" polygons for the first pole-pair number not divisible by three are achieved for p.sub.2 =3n.
The known pole-changing pole amplitude modulated three-phase windings (PAM windings) connected in delta/double wye or wye/double wye, which can be configured for any fractional pole-pair ratios, require only six terminals. However, an additional switching contact is necessary for the double wye connection. Furthermore, these PAM windings generally have in both pole-number steps an irregular m.m.f. curve with undesirable subharmonic and even-order harmonic waves.